Quite frankly teaching is no different from anything else. The more you plan the easier your lesson will be and the more your students will learn. Perhaps there are some seasoned individuals who have done the job for so long they can do it in their sleep. They might have a point. But as a rule of thumb, to advocate that you should not plan is either insecure bravado or professional irresponsibility. Neither adults nor kids are stupid. They will suss you out sooner or later. Be prepared for all eventualities.

To put it in context, I teach presentation skills in English to French business people. Rule 1, lesson 1.......Prepare!!! You have been warned.

Perhaps those TEFL course lesson plans are a bit OTT, I agree, but that doesn't stop you looking and sounding like a professional.

I think it's a really good idea to have a lesson plan. That being said, it's also a good idea not to be too attached to your lesson plans since being to rigid and dogmatic is death in the classroom. You have to be willing to admit all of your hard work and planning the night before can't compare to seat-of-the-pants, working without a net winging-it in the cold light of day.

If you have a bunch of teachers proclaiming that "planning isn't necessary", maybe observing their classes would show how that philosophy doesn't work. Just drop into their classes for an observation and the following week, let them know that there will be scheduled observations. You will undoubtedly see a marked difference.
Even after years of teaching, a plan is still necessary, even if it is a rough outline of what the class will be doing.

and I stage activities to be done that way so that the lesson is logically sequenced, from easy to difficult. It helps to create a profesional view of the lesson and to let the students know you take time to do the work you are paid for. Also, you can use the same plan with adaptations next time, you save time for preparation.

I would think all professional teachers would agree that some kind of planning is necessary to ensure everyone gets the best out of a lesson. Also it is helpful to have lesson plans to ensure continuity between each class. Lesson plans serve not only as an outline for each class but also as a record of what has been covered. Often times a point has to be revised for the students at a later date, it's good to know what has already been covered when consolidating a point.

Bottom line though, the more one is prepared the less opportunity there is to screw up!

I don't remember where I heard this quote, but it went something like,

Teaching is 10% preparation and 90% performance

I live by that quote. I believe that preparation is key to anything you do involving talking or teaching other people. And while I believe the way you teach and talk to your students is the most important, I also believe that if that first 10 percent of preparation isn't done the other 90% doesn't matter. If you have ever gone into a presentation or classroom and you could tell that the teacher wasn't prepared or the presentation/lesson was all over the place, the teacher/person loses a little bit of credibility. Students will pick up on it really quick. No matter how good an actor is, the movie will suck if the script he is following is terrible.

I always prepare what I will speak or teach, whether that preparation is in a simple note form or if it is a structured to the minute plan. And during my talks or classes it almost always changes. Depending on what your students/audience find interesting or boring, you should be able to adjust your plan accordingly. I always plan more than I think I will use, because I hate having extra time at the end of class.

Planning in general normally helps in the flow of your class, and makes things go smoother. Also, planning helps you spot potential problems that might arise before they come up in the classroom. Those other teachers you have heard from might not plan, and maybe their lessons are great, but I think for the most of us planning is in invaluable resource.